The present invention relates generally to gas generating systems and, more particularly, to gas generators used in vehicle occupant protection systems and related components, such as seatbelt pretensioners.
Gas generators used in seatbelt pretensioners are known as micro gas generators due to the relatively small size of the gas generator. Exemplary pretensioners using such micro gas generators include those described in U.S. Pat. Nos. 6,460,794, 6,505,790, 6,520,443, and 6,419,177, incorporated herein by reference. Micro gas generators generally contain an initiator including an initiator charge, an initiator retainer, and a gas generant composition which ignites and burns in response to ignition of the initiator to produce gases for actuating the seatbelt pretensioner.
In some micro gas generators, the gas generant is provided in tablet form wherein the aggregate combustion surface area is substantially higher than in gas generants provided in monolithic gas generating casts or inserts, for example. The total surface area of the gas generant in any gas generator is often optimized based on the burning rate of the respective gas generant composition. Historically, many azide-based compositions have been satisfactorily provided in tablet form given the relatively low burn rate of many azide-based compositions. With the advent of non-azide compositions, however, relatively higher burn rates often require a relatively lower aggregate gas generant surface area. Monolithic casts or inserts can be formed that effectively reduce the total gas generant surface area exposed to combustion.
In cases where the gas generant insert is in direct communication with the initiator during initiator activation, flame and ignition products from the initiator may fracture the gas generant insert. The resulting gas generant fragments will generally have a greater aggregate surface area than the intact gas generant cast. In addition, the sizes of the resulting gas generant cast fragments are unpredictable. These factors contribute to ballistic variability, making it difficult to control and predict the inflation characteristics of the gas generator.
Devices such as seatbelt pretensioners require a low initial pressurization and slow starting gas generation. The magnitude of the initial pressurization produced by activation of the initiator is directly related to initiator charge mass and surface area of the gas generant exposed to the initiator blast. Thus, one means of reducing the likelihood of gas generant insert fracture is to reduce the initiator charge mass. However, it can be expensive and/or inconvenient to reduce the initiator charge mass in order to reduce the likelihood of gas generant cast fracture. Thus, it is preferable to attempt to control the magnitude of the initial pressurization by controlling the gas generant surface area exposed to the blast. To do so, the geometry of the gas generant insert must be maintained during activation of the initiator.